1. Field of the Invention
The present invention relates to lasers producing an output beam at a single frequency with a narrow linewidth. More specifically, the present invention relates to lasers having a ring oscillator configuration and a solid state gain medium.
2. Description of Related Art
Many laser designs have been developed since the invention of the laser thirty years ago. Each different design has its advantages, and often there are tradeoffs between attainable design goals. In the design of a laser resonator, usually a tradeoff exists between output power and beam characteristics; more power usually means that the beam quality is less uniform or less single wavelength, or does not have a single polarization. An important design consideration is the output wavelength. High power outputs are provided by gain materials that lase in the infrared, such as Nd:YAG that lases at 1.06 .mu.m and Nd:glass that lases at 1.05 .mu.m. High power lasers typically operate in the pulsed mode, outputting short, high intensity bursts of light that release energy stored in the gain material. Other lasers operate in "cw", or continuous waveform that continuously outputs a beam from energy continuously applied to a gain medium.
Very high power pulsed lasers may actually comprise a number of lasers or power amplifiers connected together. A "seed", or "master" oscillator generates a single wavelength laser output which is provided to one or more power amplifiers. This configuration may be termed "MOPA" (Master Oscillator - Power Amplifier). An optimal master oscillator will produce light with a single wavelength and a high power which can be effectively amplified by the lasers or the power amplifiers. If other wavelengths are present in the output from the master oscillator, interaction between the wavelengths will lead to localized areas of intense radiation. These localized areas may lead to damage to components carrying that radiation if the intensity at that localized area is above the damage threshold of the component, such as amplifier material or mirrors or lenses. Therefore, high power lasers have been conventionally operated at an output intensity three or four times less than the damage threshold, to allow for the localized areas where the intensity may be greater. If only a single frequency were to oscillate in the laser, the laser could operate much closer to its damage threshold without damaging the optical components. Such a single frequency could be provided by a master oscillator producing a single frequency laser output.
The MOPA configuration is an example of injection, wherein a weak but well stabilized laser can control a much higher power laser. Injection concepts are applicable to a low-power laser that seeds or injects its beam into a second laser. This second laser (generally a higher power laser) may be inherently noisy and unstable, and oscillate in two or more wavelengths and transverse modes. When the low-power laser beam is injected, the higher power laser becomes much more stable.
In cw lasers, injection locking is a method of controlling a high-power cw laser with another low-power, well-controlled, stabilized cw laser. Injection locking concepts are also pertinent in the pulsed laser configurations such as the MOPA configuration, however direct application of these concepts developed for cw laser injection locking is not possible due to the transient nature of the pulse. Injection in the pulsed laser context may more aptly be termed "seeding", rather than locking because the pulse develops on its own, overwhelming the seeding injection.
The word "laser" is actually an acronym for "Light Amplification by Stimulated Emission of Radiation". Laser radiation has application in a wide variety of disciplines, such as communications, medicine, the military, research, and any other field where directed electromagnetic radiation is an advantage. The light produced from a laser has many known applications, and it is reasonable to expect that many applications of the laser have yet to be discovered. A typical laser comprises three basic elements: a resonating cavity, a gain medium, and a means to pump the gain medium.
The resonating cavity of a laser may comprise two opposed mirrors that reflect electromagnetic radiation (such as light). Other resonator configurations, such as the configuration of the ring laser, comprise three or more mirrors that reflect the light from mirror to mirror. One of the mirrors typically has less than 100% reflectivity so that a portion of the light will be transmitted and the remainder will be reflected. The output of the laser passes through this mirror, which is sometimes termed the "output coupler". For example, an output coupler may have 90% reflectivity, which means that 10% of the incident optical energy will be transmitted, and the remainder (90%) will be reflected.
The gain medium of a laser may comprise any of a variety of materials: solid materials such as Nd:YAG or Er:YAG, gases such as CO.sub.2 or Ar.sup.+, and liquids such as dye. The gain material absorbs energy from the pump, storing that energy in the form of higher energy states in the molecular or sub-atomic level.
Pumping the gain medium may be accomplished conventionally by any of a variety of devices. A lamp may be used to pump a laser that operates in the pulsed mode. A very high intensity flash of light from the lamp is absorbed by the gain material, which can then release its stored energy in a laser pulse. Flashlamps are often used to pump solid state lasers such a Nd:YAG or Nd:glass. Pumping can be accomplished in other lasers by application of an electrical current across the gain medium. Gas lasers, such as the helium-neon (HeNe) lasers or the CO.sub.2 lasers typically use electrical current to pump the laser.
Due to the smallness of the optical wavelengths, a standard laser cavity can support oscillation at many different wavelengths. A laser cavity resonator may oscillate simultaneously at several wavelengths, or "modes", or oscillation may alternate or vary between one or more of the modes competing for the gain of the laser. In some applications, the existence of several competing modes is acceptable; however, for other applications a laser that can produce a single frequency output is highly desirable. Much research is currently being devoted to design lasers whose output is a single frequency. Due to various physical constraints and design criteria, the output laser energy is generally distributed around that wavelength within a finite frequency range. The lasers that come closest to a "single wavelength" oscillate in a single mode and have a single narrow bandwidth around that single wavelength.
An etalon may be used to select the center wavelength. An typical etalon comprises a pair of opposing reflective faces that are spaced apart a distance that will support oscillation at only a specific wavelength. However, the material properties of the reflective surfaces and other material limitations restrict the ability of the etalon to produce an exact single frequency.
The ring laser configuration is known in the art to produce an output that is highly single frequency, and has been used with gain media including dye, gas and solid state materials. Dye lasers having a ring configuration are widely used and commercially available. These dye lasers produce a laser output that is often termed "single wavelength", although precisely speaking it is not a single frequency; there is some (narrow) bandwidth. As an additional feature of the ring laser configuration, only a single polarization is supported.
Despite their advantages, ring lasers have not been employed in high power applications due in part to their low efficiency and the inability to remain at a single frequency at high power. Additional problems include operating the same laser in either pulsed or cw; typically a ring laser is designed for cw operation or it is designed for pulsed operation which is generally more difficult to do.
A concern of designers of a single frequency laser is its reliability to replicate that single frequency consistently. During operation, frequency, output power, or any of a variety of characteristics may waver, due to a variety of complications such as variations in the gain due to input power fluctuations. For example, the center frequency of a laser pulse may be at 1.053001 .mu.m, the next pulse may have a center frequency at 1.053003 .mu.m and continue to waver erratically. Injection locking of a ring laser by a smaller, external, well-controlled laser can produce a single frequency of operation; however, this requires servo control of the cavity length of the ring laser to that of the external laser.
In a pulsed laser it is desirable to produce a series of pulses as nearly identical as possible. Performance criteria for the pulsed laser includes energy output, rise time, temporal stability, amplitude stability, and single mode (frequency) content. Energy output is a measure of the intensity of the pulse integrated over the beam area. Rise time is a measure of how quick the pulse rises from minimum amplitude to approximately full amplitude. Temporal stability is a measure of predictability of the rise time between pulses; it is desirable to have this figure as small as possible, down in the tens of nanoseconds. Amplitude stability is a measure of predictability of the amplitude from pulse to pulse; it is desirable to have this figure down to a few percent.
For many high power applications, the gain medium of the power amplifiers is Nd:YAG which lases at 1.06 .mu.m. or Nd:glass which lases at 1.05 .mu.m. Thus, the master oscillator must produce this same wavelength. Dye lasers are unsuited as the master oscillator around 1.05 .mu.m. Although dye lasers are tunable by conventional techniques to provide any one of a number of different output wavelengths, existing dye lasers cannot be tuned to attain the 1.05 .mu.m wavelength necessary for seeding solid state glass amplifiers.
Thus, it would be advantageous to provide a master oscillator that has an output at a frequency around the peak gain of solid state laser materials, with a spectrum containing a single sinusoidal frequency and a high output power (8 watts continuous or &gt;2 mJ/pulse in pulsed mode). A small frequency range for the pump laser is particularly desirable in the master oscillator of high power lasers, to minimize the possibility of damage to optical components caused by interference and reinforcement between the various wavelengths in the beam. Such a master oscillator can be used as a single frequency seed for high power laser systems for military tracking and other applications such as laser fusion. Furthermore, the master oscillator can be used to seed a high power laser which powers an x-ray source applicable in lithography.
Nd:YLF as a gain material has been used in high power pulsed oscillators that have a linear configuration. It has been suggested that it can be used as the gain medium in a ring laser configuration.